I've been having this issue where if my heat inside is not on I leak coolant from this little cylinder right outside the firewall. I don't know what it is or if there's an easy fix rather than replacing the part. Summers comin up fast and it really sucks to have to have my heat on when its hot out. Any suggestions? What is that little can anyway?

AZ-Jeff
I don't own an XJ any longer, but I still think they are the best of all the Jeep products ever made. My XJ was my favorite vehicle in my 50+ years of driving.
So...I stick around and give advice.

If it's the part with 4 hoses attached to it, that is the diverter valve for the heater.
If yours is plastic, it may be cracked (I looked at mine too hard one day and it fell apart!) or the hose clamps might be loose.
(I just googled it and an 89 should be metal...)
If the clamps are tight then just replace the part.